Thermal treatment of articles is frequently used in industry to alter or develop desired properties of the material comprising the article. Rapid heating or cooling of the article to a given temperature may be needed, or a slower, regulated rate of temperature change may be required. Also, holding an article at a given temperature for a period of time may be desired.
An important application of thermal treatment is the hardening of carbon and alloy steels. This is commonly accomplished by heating the steel article to a temperature of 1500 to 1700.degree. F., where the alloy is transformed to the austenite phase, and then rapidly cooling or quenching the alloy. The composition of the alloy, the rate of cooling and the temperature levels attained determine the phases, and hence the properties of the final product.
Quenching may be accomplished in a number of ways. In spray quenching, the hot article is sprayed with a cool liquid. In gas quenching, the article is cooled by a flow of gas or vapor. A variation is fog quenching where a gas or vapor flow carries fine liquid particles into contact with the article. In immersion quenching, the article is immersed in a liquid bath such as water, oil, brine, polymer solution, liquid cryogen, or molten salt.
Each of these quenching methods, although successfully employed, have some undesirable characteristic. Liquid quenchants often leave a layer of deposit which must be removed. Polymers, and oils degrade with usage and age and must be replaced. Molten salts degrade with usage and present an environmental disposal problem. Most of the liquids boil and exhibit complex cooling behaviors (e.g. liquid phase convection, vapor phase convection, nucleate boiling) which are difficult to predict. In addition, the cooling behavior of each medium varies with the degree of agitation and the position and orientation of the article with respect to other articles in the medium. Further, the cooling performance may change due to thermal degradation, contamination or depletion of a component by drag out or distillation.
The use of fluidized beds for quenching obviates many of the problems associated with liquid quenchants. There is little or no cleaning of the article needed after quenching in a fluidized bed. The particles in a fluidized bed do not degrade rapidly with time or usage so that the cooling rates remain unaffected over long periods of time. The heat transfer mechanisms in a fluidized bed are dominated by the properties of the gas film on the article and the particles, and remain approximately constant throughout the quenching temperature range. Thus the quench rate of a fluidized bed is reproducible, can be adjusted within limits, and can be provided over a wide temperature range.
In spite of these advantages, fluidized beds have not found wide application for quenching because quench rates in fluidized beds approaching those obtainable in liquid baths have not been readily attainable in the past. The state of the art in fluidized bed quenching has been reviewed in an article entitled "Fluid Bed Quenching of Steels: Applications are Widening" published by M.A. Delano and J. Van den Sype in Heat Treating, December 1988. This article is hereby incorporated by reference, and some of its content is summarized following.
To employ a fluid bed to adequately harden an alloy steel article having a significant cross section (e.g., one-half to two inches thick) it is necessary to develop heat transfer coefficients similar to those obtainable in a well-agitated oil, namely 250 to 300 BTU/hr-ft.sup.2 -F.degree.. This requires optimizing the fluid bed parameters of particle size, particle material, fluidizing gas composition, and fluidizing gas flow rate.
Fluidization of a mass of particles occurs when the particles are caused to separate from continuous contact with each other, move about and collide randomly with each other and confining boundaries. This can be accomplished, as is known, by vibrating the boundaries confining the bed, in particular, the bed support. Alternatively an article introduced into the bed for processing may itself be vibrated, thereby fluidizing the particles adjacent to the article. A method of fluidization that is more readily accomplished in commercial practice is passing a flow of fluid upward through the bed. The lowest flow at which the bed has expanded and the particles are suspended, move about and randomly collide is denoted the minimum fluidizing flow. Fluidization of particles by fluid flow of course can be combined with vibrofluidization, but a considerable increase in apparatus complexity is necessary. Thus fluidization by an upward flow of gas provides a means for exchanging heat between an article, the bed particles and the fluidizing gas, and is viable for thermal treating and quenching.
Particle diameters considered for use in fluidized bed quenching range from 20 to 2000 microns. Highest heat transfer coefficients in the bed are obtained as particle size is diminished to about 30 microns. When still smaller particles are fluidized by gas flow, the type of fluidization changes from a bubbly to an aerated character, and the heat transfer coefficient in the bed decreases Precipitously. To achieve high coefficients without risk of approaching the change in fluidization character and to maintain the loss of particles from the bed at an acceptable level, an operable lower Particle size is about 50 microns, and a preferred lower particle size is about 70 microns.
Particles that may be used in fluidized beds are metal oxide particulates such as aluminum oxide, chromium oxide, iron oxide and titanium oxide; refractory particulates such as silicon dioxide, mullite, magnesite, zirconium oxide and forsterite; and elemental particulates such as iron, copper, nickel and carbon. With the variety of materials and variations in porosity that can occur, the apparent density of the particles used in fluidization can be varied over a range of from 0.3 to 20 grams per cubic centimeter. Aluminum oxide in the form of alumina because of its inertness, high heat capacity and reasonable cost is a preferred bed constituent.
The thermal conductivity of the fluidizing gas has a major effect on the heat transfer coefficient in the bed--higher conductivities providing higher coefficients. Thus hydrogen and helium, which have thermal conductivities of 0.0975 and 0.0805 BTU/hr-ft.sup.2 -F.degree./ft, respectively, at room temperature, are high thermal conductivity gases while nitrogen and air, which have a thermal conductivity of about 0.014 Btu/hr-ft.sup.2 -F.degree./ft, are low thermal conductivity gases by comparison. Because of the flammability of hydrogen, helium is a preferred high thermal conductivity fluidizing gas. In some instances, lower heat transfer coefficients are acceptable so that less costly, low conductivity gases such as nitrogen or air are usable. Also mixtures of low and high thermal conductivity gases have utility. Gases, or mixtures of gases, with thermal conductivities equal to or exceeding 0.05 BTU/hr-ft.sup.2 -F.degree./ft at room temperature will be denoted high conductivity gases for convenience. Gases with lower conductivity will be denoted low conductivity gases.
The heat transfer coefficient in a fluidized bed increases with fluidizing gas flow rate from the minimum fluidizing flow until a maximum coefficient is reached over a range from five to fifteen times the minimum fluidizing flow. Beyond this range the coefficient gradually decreases due to the increased fraction of bubbles in the bed. Towards the high end of this flow range, particles are carried out of the bed in increasing amounts and the cost of the fluid used is a consideration. Thus preferred rates range from five to ten times the minimum fluidizing flow.
Pursuant to common Present practice, with a preferred combination of 70 micron diameter particles of alumina fluidized by helium at ten times the minimum fluidization rate, a heat transfer coefficient of 240 BTU/hr-ft.sup.2 -F.degree. is developed. The quenching performance in this bed would be somewhat inferior to that of a well agitated oil bath where a coefficient of 280 is obtained. In addition, an undesirable amount of alumina material would be carried out of the bed by the fluidizing helium. Exposure of personnel to this effluent would be an unacceptable health hazard. The cost of the helium used would also be appreciable. To achieve a coefficient of 280 Btu/hr-ft.sup.2 -F.degree. in the fluid bed, a flow of fifteen times the minimum fluidizing flow would be necessary. This would lead to still greater carry-out from the bed and still greater fluidizing gas cost.
While carry-out from the bed would be reduced by increasing the bed particle size, this is not a viable option. The heat transfer coefficient would decrease and the gas consumption would need to increase markedly.
Therefore an object of this invention is to achieve high heat transfer coefficients in a fluidized bed with reduced fluidizing gas flow rates and with reduced carry-out of bed particles.
Another object of this invention is to provide an apparatus and improved processes for quench hardening articles having thicknesses at least up to two inches.
Still another object of this invention is to provide a fluidized bed which will have greater utility in temperature treating materials.
Other objects and a fuller understanding of the invention may be had by referring to the following description and claims taken in conjunction with the accompanying drawings.